This invention relates to fuel cell and electrolysis cell stacks. It particularly relates to improvements in molten carbonate fuel cell stacks directed to delay and control the effects of electrolyte migration.
Molten carbonate fuel cells and stacks of such cells are well known and described in various prior publications and patents. For example, U.S. Pat. No. 4,478,776 to Maricle et al. and U.S. Pat. No. 4,411,968 to Reiser et al. illustrate typical fuel cells and stacks of such cells. Porous, sintered nickel-chromium anodes and porous nickel oxide cathodes are disposed on opposite major surfaces of a porous electrolyte matrix. A matrix of such as lithium aluminate (LiAlO.sub.2), or other inert ceramic is filled with molten alkali metal carbonate electrolyte, (eg. Li.sub.2 CO.sub.3 /K.sub.2 CO.sub.3) in each fuel cell of a stack. Stacks with several hundred fuel cells are contemplated in a typical power supply. Severe electrolyte redistribution has been observed even in experimental stacks with substantially fewer cells than that expected for an operational power supply.
The exact mechanism by which the electrolyte migrates is not clearly understood. Nonetheless, it is known that the cells near the negative end of the stack become flooded while the cells towards the positive end of the stack become depleted or dry of molten electrolyte. It is known that an electrical shunt current through the manifold gasket causes electrolyte migration towards the negative end of the stack. The flooding and depletion of electrolyte severely impair the performance of the affected cells and greatly increase the overall resistance of the stack.
In prior fuel cell stacks, the manifold gasket was selected to provide a good seal against gas leakage. Such a seal was porous, with small pores sized to significantly fill with molten electrolyte. Although high electrolyte content minimizes gas leakages, it unfortunately promotes electrolyte migration.
The efforts to eliminate electrolyte redistribution have not been completely successful since most porous gaskets, wetted with electrolyte will conduct the electrolyte to the negative end of the stack. If the fuel cell stack is arranged with the positive end at the bottom and the negative end at the top, gravity will resist migration of electrolyte towards the more negative cells. However, this is not sufficient to prevent redistribution of electrolyte towards the upper cells.
Therefore in view of the above, it is an object of the present invention to provide a fuel cell stack with improved electrolyte migration control.
It is a further object to provide fuel cell stack improvements for limiting the rate of electrolyte migration.
It is also an object of the present invention to provide fuel cell stack improvements to delay the effect of electrolyte migration beyond the normal operating cycle of the fuel cell stack.